1. Field of the Invention
The present invention relates to receiving technique of digital multiplex broadcasting.
2. Description of Related Art
As to satellite broadcasting for a movable body, a terrestrial wave is transmitted by means of a terrestrial wave repeater for a satellite wave interpolation, in addition to a satellite wave transmitted from a satellite. In this case, a TDM (Time Division Multiplexing) signal is utilized for the satellite wave. On the other hand, since the terrestrial wave requires a multiplex system, the multiplex systems such as an OFDM (Orthogonal Frequency Divisional Modulation) and a CDM (Code Division Multiplexing) are utilized for the terrestrial wave. As a result, both the satellite wave and the terrestrial wave of which systems are different are mixed, and a receiver receives a multiplex signal of a satellite wave and a terrestrial wave.
Generally, a receiver includes an AGC (Auto Gain Control) circuit which controls gain of a received signal, and such a receiver that receives the multiplex wave is formed to separately execute an AGC for each of broadcasting systems. Namely, the receiver separately executes the AGC to the broadcasting wave by a terrestrial wave digital broadcasting (hereafter it is called “terrestrial wave”) and the broadcasting wave by a satellite broadcasting (hereafter it is called “satellite wave”) through separate AGC circuits. Specifically, the receiver separately detects a signal level of each system from the multiplex signal, generates the AGC signal and separately executes the AGC. Accordingly, the receiver demodulates the received signal of each system, mixes both signals and supplies them to a decoder.
FIG. 1A shows frequency bands of the satellite wave and the terrestrial wave. Generally, the satellite wave is located neighboring to the terrestrial wave on a frequency axis in many cases. An example in FIG. 1A is the case of a certain system, in which the satellite waves are located neighboring on both sides of the terrestrial wave on the frequency axis. As for the terrestrial wave, the receiver extracts only terrestrial wave signals through a filter for the terrestrial wave which has a pass frequency band PB1 corresponding to a terrestrial wave frequency band. On the other hand, as to the satellite wave, if filters which each frequency band of the satellite wave passes through are separately prepared, a cost of the whole receiver will increase because the filters for plural frequency bands are needed. Thus, as shown in FIG. 1A, it is common that the filter for the satellite wave having a pass frequency band PB2 which the frequency band including both the terrestrial wave and the satellite wave can pass through are utilized in order to cut down the cost of a front-end of the receiver.
Since the satellite wave is transmitted from an artificial satellite, its electric field strength is small and the satellite wave is received at a roughly constant level. On the contrary, the electric field strength of the terrestrial wave is considerably varied in accordance with a range from a transmitting antenna to a receiving point. FIGS. 1A to 1C show varying examples of a received signal spectrum according to the range from the terrestrial wave antenna. As shown in FIG. 1A, a terrestrial wave component level of the multiplex wave which is received at a near range from the terrestrial wave antenna is large. However, as shown in FIG. 1B, the terrestrial wave component level of the multiplex wave which is received at a middle range from the terrestrial wave antenna is not so large. As shown in FIG. 1C, the terrestrial wave component level of the multiplex wave which is received at a far range from the terrestrial wave antenna is very small. Like those, though the signal level of the satellite wave is comparatively stable, the signal level of the terrestrial wave varies considerably dependently on the range from the transmitting antenna. Further, as to the terrestrial wave, effects of a fading and a multipath are also added to the signal level by surroundings. Accordingly, the signal level of the terrestrial wave is quite variable in comparison with that of the satellite wave.
As described above, in the case that a filter passing all frequency bands is utilized as the filter for the satellite wave as shown in FIG. 1A, an AGC circuit for the satellite wave executes the AGC based on the received signal in the frequency band including the terrestrial wave. Thereby, a problem described below can happen.
In the case of this example, since the satellite waves are located on both sides of the terrestrial wave, the terrestrial wave level is larger than the satellite wave level at the near range from the terrestrial wave antenna as shown in FIG. 1A. Therefore, in accordance with a large level of the terrestrial wave signal, the AGC circuit for the satellite wave executes the AGC in order not to generate a distortion of the level by a saturation of the terrestrial wave signal. Thus, the attenuation is also carried out to the satellite wave signals. As a result, the satellite wave which essentially needs to be received is also attenuated, and receiving the satellite wave may be difficult.
Also, as explained above, the terrestrial wave is easily affected by the fading and the multipath by the surroundings. Once the terrestrial wave level becomes unstable by the effects of the fading and the multipath, the AGC circuit for the satellite wave performs gain adjustment according to the variation. As a result, it sometimes happens that the level of the satellite wave cannot be controlled accurately.